[chromecast] mixer_unittest only test 2 output channels.

Bug: internal b/77653377
Test: on device
Change-Id: If07a4b9af7a97779c08817fbdd4ba7b9740cdd15
Reviewed-on: https://chromium-review.googlesource.com/1043390
Reviewed-by: Stephen Lanham <slan@chromium.org>
Commit-Queue: Gaofeng Huang <gfhuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556061}

chromecast/media/cma/backend/stream_mixer_unittest.cc

@@ -443,13 +443,13 @@ class StreamMixerTest : public testing::Test {
     mixer_ = std::make_unique<StreamMixer>(std::move(output), nullptr,
                                            base::ThreadTaskRunnerHandle::Get());
     mixer_->SetVolume(AudioContentType::kMedia, 1.0f);
-
     std::string test_pipeline_json = base::StringPrintf(
         kTestPipelineJsonTemplate, kDelayModuleSolib, kDefaultProcessorDelay,
         kDelayModuleSolib, kTtsProcessorDelay, kDelayModuleSolib,
         kMixProcessorDelay, kDelayModuleSolib, kLinearizeProcessorDelay);
     auto factory = std::make_unique<MockPostProcessorFactory>();
     pp_factory_ = factory.get();
+    mixer_->SetNumOutputChannelsForTest(2);
     mixer_->ResetPostProcessorsForTest(std::move(factory), test_pipeline_json);
     CHECK_EQ(pp_factory_->instances.size(),
              static_cast<size_t>(kNumPostProcessors));